This invention relates to a screw compressor and, more particularly, to a structure which prevents a slide valve from contacting screw rotors.
Details of an example of an ordinary type of screw compressor are disclosed in U.S. Pat. No. 3,088,659, in which, as shown in the accompanying FIG. 3 which illustrates essential parts of this example, a pair of male and female rotors 2 and 3, which compress gas in a casing 1, are accommodated in the casing, and a slide valve 4 which can move axially while maintaining small gaps between itself and the outer peripheries of the rotors 2 and 3 is disposed inside the casing 1, the rate of gas flow bypassed to a gas suction port during compression being regulated by axially moving the slide valve 4.
When, in this type of screw compressor, the slide valve 4 moves in the axial direction, it receives radial forces in response to pressure fluctuations generated between the slide valve 4 and the two rotors. As a result, lip portions 1a and 1b of the casing 1 are opened by the radial forces and the heat applied to the slide valve 4, and their function restraining the slide valve 4 is thereby lost. The slide valve 4 is therefore brought into contact with the outer peripheral surfaces of the two rotors, and the outer peripheral surfaces of the rotors become worn, thereby causing problems of a reduction in performance and the generation of abnormal noise.
To eliminate these problems, the above patent specifies that a shaft which guides the slide valve is thickened to minimize the extent of movement thereof in the radial direction due to pressure fluctuations. This construction is suitable for a big, large-capacity screw compressor having a comparatively bulky casing, but it is not suitable for a comparatively small screw compressor such as that disclosed in U.S. Pat. No. 3,796,526 in which a motor and screw motor portions are incorporated in one casing. Another possible solution involves a T-shaped guide block which is disposed on the side of the outer circumference of the slide valve, thereby limiting the radial movement of the slide valve. However, it is difficult in practice to manufacture this compressor in terms of the workability and accuracy of the groove into which the guide block must be fitted.
As described above, the conventional known arts cannot consider the workability and accuracy of the fitting groove for the guide block, and it is therefore difficult to work and form the groove to a high enough accuracy to produce the compressor. In a construction in which the guide block is provided as a member which is separate from the casing, the accuracy of the assembled state, such as inclination and/or bending, is reduced, and the guide member is not easy to assemble and cannot be disposed so as to prevent the rotation thereof. Therefore, a compressor of this construction is even more difficult to produce.